1. Field of the Invention
The present invention relates to an optical disk apparatus and, more particularly, to a tracking-servo control technique of a light beam for writing or reading data to or from an optical disk.
2. Description of the Related Art
With the increasing needs for high speed data processing performance of digital systems, optical disk apparatuses have received a great deal of attention as external data storage devices which can store large amounts of data. In recent years, an application range of sample-servo formatted optical disks has been increased in addition to pre-groove optical disks which are conventionally known. Since the sample-servo formatted optical disks do not have a groove for guiding a laser light beam to a track or tracks, higher-density data storage can be advantageously realized.
In order to write or read data to or from an optical disk, high-precision servo control of a laser pickup section of an optical disk apparatus is required so that a spot of a laser light beam focused on the surface of the optical disk accurately traces a track of the optical disk. Otherwise, effective spot energy to the track and/or effective light reflection from the track cannot be obtained, thus degrading efficiency of a writing/reading operation of data.
According to a presently available servo control technique of a laser light beam, servo control pits are typically formed for each track of the sample-servo formatted optical disk. Each pit normally has a circular or elliptic shape smaller than a beam spot. The pits include clock pits, a pit central point of each of which is positioned on the central line of the width of the corresponding circular-shaped track, and first and second pits (to be generally referred to as "wobbled pits") each positioned at the both sides of the corresponding track. These pits are repeatedly formed along the track circumferences in each servo region at preselected intervals.
When a positional relationship between a moving laser beam spot (which may be normally considered as a circle) which is emitted from a pick-up section of the optical disk apparatus and focused on the optical disk surface and these pits is optically detected, a "positional shift" of the spot can be specified. For example, when monitoring of the coincidence between the moving spot and certain clock pits is continued on the basis of the light reflected thereby, it is determined whether the beam spot accurately traces the tracks. In addition, using a difference between components of the light reflected by a pair of wobbled pits, an eccentric shift (or an "undulating" shift) between a locus of the beam spot and the track can be detected. In a principle, when a relative position of the pick-up section with respect to the track is adjusted to compensate for the detected shift, tracking control of the optical disk can be performed in real time.
Such an arrangement, however, has a drawback that the precision of track-accessing control cannot be improved for the following reason: a track-crossing direction of beam spot on the optical disk cannot be detected precisely be only detecting a difference between reflected light components from each pair of wobbled pits. In order to detect the track-crossing direction at high precision, if a specially designed circuit, such as a linear-scale circuit, is provided additionally, the overall circuit configuration will be complicated undesirably.